How Nitrogen-Fixing Plants Can Perk Up Your Garden

Legumes, like peas, can help you reduce your reliance on fertilizers. rodimov/Shutterstock

Are you an ecologically sensitive gardener? Would growing things that give back to the environment make your heart skip a beat? If the answers are yes, then get ready to fall in love with the nitrogen-fixing plants of the legume family.

Legumes — beans, peas and non-edible relatives such as clovers — give back to your garden because they have a symbiotic relationship with a soil bacteria. This special relationship allows them to convert atmospheric nitrogen (N2) into ammonium nitrogen (NH4), which they release into the soil. This is a big deal for tomatoes, broccoli, peppers and other common plants in backyard vegetable gardens. That’s because most plants can’t absorb atmospheric nitrogen, which is an inert gas. They need to absorb nitrogen, an essential building block for all plants, from the soil through their roots.

The way for home gardeners to take advantage of the organic nitrogen-fixing capabilities of legumes and reduce reliance on chemical fertilizers is not to grow food crops such as beans and peas, said Julia Gaskin, a sustainable agriculture coordinator at the University of Georgia. Rather, she said, you should grow legumes as cover crops in advance of food crops. “Cover crops are things that we plant in the garden to promote ecosystem services,” said Gaskin. “In the case of legumes, they provide nitrogen for vegetable crops.”

Here is a guide to understanding how nitrogen-fixing plants work their magic and how to show them the love in your garden.

How nitrogen fixing works

Before planting a cover crop, Gaskin said it helps to understand how legumes fix nitrogen into the soil. The bacteria with which legumes have a symbiotic — mutually beneficial — relationship are Rhizobia bacteria, little bacteria that infect legume roots and exist naturally in soil, said Gaskin. “The bacteria are able to do this miraculous conversion by taking nitrogen gas and converting it into a chemical form, ammonium that plants can use. In return, the plant supplies the bacteria with carbohydrates, which gives them energy to function.”

One of the key goals with cover crops, she said, is to keep a living root in the soil at all times. “It’s how we keep that whole ecosystem down there in the soil growing. Roots exude carbohydrates and other things, and they keep those little micro-organisms down there alive and healthy.”

Before planting cover crops, Gaskin urges home gardeners to take an extra step she likens to an insurance policy to ensure cover crops fulfill their nitrogen-fixing role. “We recommend that you inoculate your legume seeds with these Rhizobia bacteria. Then you know [the bacteria is] right there when the seeds germinate and it’s ready to infect the root.” The inoculant is often available where cover crop seeds are sold. But, Gaskin added, it’s important to remember when you buy the inoculant that it is a living bacteria. “Don’t go buy a bag of inoculant and throw it on the dashboard of your car and go run a bunch of errands,” she advised. “High heat will kill the bacteria.” It should be stored in a cool place such as a refrigerator until you are ready to use it. While it may seem unnatural to store bacteria with food, it is not going to “escape” and cause harm.

Growing cover crops also requires gardeners to do something else that is unnatural: kill the plants before they set seed. Legumes need nitrogen they have obtained from the Rhizobia to produce seeds. The nitrogen that has been “fixed” from the air into the ground is used to make proteins in the seed. To get the most nitrogen form a cover crop, it needs to be killed before it sets seed. That’s why legume food crops don’t supply much nitrogen for subsequent crops.

Choosing cover crops

Popular winter cover crops include crimson clover, which Gaskin called the best clover for the South, red clover, which she said is often used in other regions, Austrian winter peas and hairy vetch. The latter, she said, comes with something of a warning. “In the South, hairy vetch tends to become a weed if you don’t kill it before it sets seed.“

Cover crops can also be grown in the summer. Sun hemp is a topical legume that can be planted during warm months. It produces quite a bit of nitrogen in 60 to 90 days. Forage soybeans and cowpeas are also popular summer cover crops.

A summer legume cover crop is something that can enhance the production of a fall broccoli crop, Gaskin said. She suggests planting cowpeas at the end of May or June and tilling those in August. When transplants of broccoli, which demands a lot of nitrogen, are set out later in the fall, the cowpeas will supply much of the nitrogen the broccoli requires, Gaskin said.

Whenever you plant cover crops, Gaskin said, it’s important to think about something she said home gardeners often miss: How are you going to manage your cover crop? “It’s easy to hand broadcast many of the cover crop seeds and rake them in,” Gaskin said. But, she pointed out, some companion cover crops, such as cereal rye, might have so much biomass that a home gardener has trouble killing it and working it into their garden. “You have think about ‘How am I going to kill this thing! How am I going to till it in?’ before you get out there and plant something that may give you more biomass than you can manage with a little tiller.“

Crimson clover is a solid cover crop choice for gardeners in the South. (Photo: Cathleen A Clapper/Shutterstock)

Managing cover crops

There are several ways to kill cover crops organically. “Most people mow and till in their cover crop,” said Gaskin. A creative way to kill a cover crop is to cover it with cardboard or plastic and smother it. More conventional gardeners could use a herbicide. “How you kill a cover crop just depends on your how you want to garden,” Gaskin said. Whether you kill cover crops organically or not, the critical thing is not to pull them out of the ground and put them on a compost pile. “They are not going to do their job unless they are left on the surface or incorporated into the soil,” said Gaskin.

When you kill a cover crop depends on several factors. One of those is whether you are leaving the biomass on the surface to act as a mulch or whether you are incorporating it into the soil. If you are tilling it in, it could take three weeks or longer for the cover crop to break down. The soil also needs to have moisture and to be warm for the biomass to begin decomposing. “If you mow something and turn it under in March when the ground is cold, it’s not going to break down very quickly,” Gaskin said. “If you’re leaving things on the surface and you’re transplanting into it, then you want for it to at least dry out a bit so it’s not green. Then you just dig a little hole and transplant your tomato in there and use the cover crop like a mulch to help suppress weeds. So there’s a lot different directions and methods you can use.”

If you plant a cover crop ahead of a crop with small seed, such as lettuce, you’ll have to allow plenty of time for the cover crop to break down. “You don’t want clumps of cover crop where you’re trying to plant small lettuce seeds,” said Gaskin.

Mixing and matching legumes and grains

Cereal grains such as cereal rye, wheat, oats and barley are legume companions that can be used as cover crops, although they aren’t nitrogen-fixing plants. Grains tend to be fairly deep rooted. They are called scavengers because their roots bring nutrients back up to the surface and into their stems and leaves. When the plants are killed, they put those nutrients back into the root zone for the next vegetable crop as they decompose. “Cereal rye is particularly good at this,” Gaskin said. So are oats and millet, she added.

“One of the things I love,” Gaskin said, ”is mixing grains and legumes. One of my favorite cover crops is a mix of oats and crimson clover. With a pure legume cover crop, many times the nitrogen is released the first month after you turn it in. If you add a little grain, it helps to release the nitrogen over the summer growing season. So you can mix and match some of these things.”

Despite being considered a weed, white clover is a good nitrogen fixer. (Photo: Grigorii Pisotsckii/Shutterstock)

Ornamental gardens and lawns

Because legumes release nitrogen as they are decomposing, ornamentals in the legume family fix little if any nitrogen in perennial flower beds, Gaskin said. However, she added, if you are willing to have a lawn that doesn’t look like a manicured putting green, white clover is a nitrogen-fixing plant that can be added to a fescue lawn.

White clover is a perennial and it fixes nitrogen in the soil, Gaskin said. “When you mow a lawn with white clover in it, the root system of the clover prunes back because there’s not enough carbohydrates being fixed by sunlight to support the grass that’s growing above ground. When the roots die back they release a little bit of nitrogen.”

“A lot of people think white clover is a weed,” Gaskin said. “it just depends on what you want. Besides, it’s wonderful for the bees.”

All plants give back

One other thing to remember, Gaskin said, is that all plants give back in some way, whether it’s through the beauty of flowers or the pollinators they support or in a more personal way.

She recalled how she grew cut flowers for her daughter’s wedding this summer – zinnias, purple cone flowers, rose yarrow and black-eyed Susan. “To get them to keep blooming, I had to keep harvesting the flowers. A friend of mine and I would take them to the food bank and make bouquets for folks. You would not believe how much that meant to people. To have something that would be just beautiful handed to them at a time when most of them are struggling.”